Tall garden phlox

Quick facts

Tall garden phlox have long-blooming, colorful flowers that pop in any garden.

Tall garden phlox are hardy and vigorous when grown in the right conditions.

These perennial flowers are often used as background plants in narrow borders or in groups between taller and shorter plants in a wide border.

A mixed planting of tall garden phlox.

Count on tall garden phlox (Phlox paniculata hybrids) to provide a colorful summer display in perennial gardens, blooming as much as six weeks or more. Some cultivars begin blooming in mid-summer, others not until late August. Most tall garden phlox grow two to three feet, with some slightly taller.

Garden phlox are available in a wide range of pink, rose, red, lavender, purple, orange and white, along with bi-colors that have an "eye" in the center of each flower or a contrasting margin. Cultivars range from softest pastels to electric, "knock-your-socks-off" brilliant blooms. In addition to their visual appeal, many tall garden phlox are sweetly perfumed.

Tall garden phlox needs sun to bloom.

Site and soil

Tall garden phlox grow best in full sunlight, but will tolerate light shade

Shading decreases both bloom quantity and quality, while it increases disease problems

They should not be planted near trees and shrubs that compete for moisture and nutrients

Avoid placing them next to fences or walls where air circulation is poor because this is an invitation to disease problems

Choose soils that drain well, yet retain adequate moisture

A soil test every 3 to 5 years is helpful in determining the need for soil amendments and fertilizer

Planting

Tall garden phlox are typically purchased as potted plants from nurseries, garden centers, or local farmers' markets. Make sure you get Phlox paniculata because there are other phlox species that, while very lovely, will have different growth habits and growing requirements than what we describe here.

Potted nursery plants may be transplanted into the garden throughout the growing season, right up to early autumn.

Buy bare root plants from mail-order nurseries in spring.

Avoid purchasing bare root plants in the fall. Fall-planted specimens are more subject to frost heaving.

Mail-order plants might arrive too late in fall to be planted safely.

When planting, set the crown of each plant about an inch and a half below the soil surface for both potted and bare root plants.

Incorporate generous amounts of organic matter into any soil before planting. And it's an absolute necessity if the site has sandy soil.

Composted yard waste, peat moss, or well-rotted manure all work well to improve soil structure.

Mulch fall-planted phlox with four to six inches of hay or straw, chopped leaves, or pine needles as soon as the soil freezes.

After the first year fall mulching is less critical, though it's always a good idea.

Mulch not only protects plants from winter's cold, it helps prevent premature soil warming that could result in early spring growth, before weather is reliably mild.

Transplanting and dividing

Most phlox need to be divided every two to four years if they're growing well.

The best time to do divide and transplant is in spring, before new growth begins.

Each division should consist of three to five vigorous shoots with roots attached.

When planting, position them as you would new plants, with the crowns an inch or so below the soil surface.

Space them 18 inches apart to ensure adequate air circulation.

How to care for tall garden phlox

Keep soil moist by watering thoroughly on a regular basis

Water the soil rather than phlox foliage to aid in disease prevention

If you must water overhead, water early in the morning so plants will dry rapidly in the sun

Spread two to three inches of mulch after the soil warms in mid- to late June to keep the soil cool and damp, and to reduce weed growth

Once stems are about six inches tall, eliminate all but five or six stems per plant, then pinch back the growing tips of those remaining stems (you'll find they become more robust, produce larger clusters of flowers, and are less bothered by powdery mildew)

A spring application of manure or compost is beneficial each year

If not using manure or compost, you can apply a light application of 10-10-10 fertilizer as new growth emerges each spring, and then again just before plants begin to flower

If your phlox produces seeds, the resulting seedlings will not resemble the parent and will usually turn out to be pale magenta. To keep your plants the color you chose initially, it's important to "deadhead" or remove clusters of faded flowers so mature, viable seeds are not dropped to the ground.

Some newer phlox cultivated varieties are sterile, but even then, deadheading is a good idea because it results in more attractive plants. And sometimes if you fertilize adequately, phlox may put out an unexpected second smaller flush of blooms.

Common problems

In Minnesota, the most common problems of tall garden phlox are powdery mildew, a fungal disease, and spider mites.

|

Powdery mildew can be a destructive disease on tall garden phlox. It appears as disfiguring, powdery white spots on the foliage. When these spots merge, they can nearly obliterate any remaining green tissue, moving onto the flowers as well.

Maintaining good air circulation will help reduce powdery mildew problems, but the best way to avoid the disease is to choose disease resistant cultivars in the first place, then make sure they're neither crowded nor shaded when you plant them.

If necessary, you could begin a fungicide spray program using sulfur or chlorothalonil (sold as Daconil 2787) at the earliest signs of infection.

Phlox may also develop other fungal leaf spots, particularly on lower leaves. However, they are rarely serious enough to warrant treatment.

CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.

Spider mites are the most troublesome "insect" pests of phlox. (Mites are not true insects, but are arachnids, similar to spiders.) They insert their sucking mouthparts to remove liquids from phlox leaves, causing pinprick yellow discoloration of the foliage, eventually turning leaves dry and brown.

Spider mites are generally worse in hot, dry summers than when it's cooler or more moist. Vigorously growing plants are better able to tolerate mite feeding, so make sure phlox are watered and fertilized adequately to insure their vigor.

An insecticidal soap spray can be moderately effective, particularly if you catch the mite infestation in its early stages.

If the mite problem is severe, check your local garden center for Kelthane (dicofol), a miticide labeled specifically for use on ornamentals or flowering perennials.

Apply any spray very early in the day, when temperatures will remain cool for several hours.

Spraying during the heat of the day is more likely to damage foliage.

CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.

How to choose tall garden phlox

A cultivated variety, or 'cultivar', is a plant selected from the wild or intentionally bred that differs from the typical member of the species from which it was selected or bred. Cultivars of tall garden phlox have been selected for various flower colors and heights.

Dozens of different phlox cultivars will grow in Minnesota, but many are quite disease-prone. In their book Growing Perennials in Cold Climates (Contemporary Books, 1998), authors Mike Heger and John Whitman rated the following phlox as "five star" perennials — the best available on the market today.

Here, with their permission, is a list of those plants. All are hardy to -40F degrees.

'Bright Eyes' - pink flowers with red eye, 36 inches tall

'David' - white flowers, 36 inches tall

'Eva Cullum' - pink flowers with red eye, 30 inches tall

'Franz Schubert' - lilac flowers, 30 inches tall

'Russian Violet' - violet purple flowers, 30 inches tall

'Sir John Falstaff' - salmon pink flowers, 30 inches tall

'Starfire' - cherry red flowers, 30 inches tall

'World Peace' - white flowers, 42 inches tall

Other newer cultivars include the Early Start series, which bloom earlier and have a compact form, and the Flame series, which have a long bloom time and compact form. Both the Early Start and Flame series are available in several different colors.